The present invention relates to steam generating boilers and, more specifically, to a method of improving the response to changes in load demand of boilers equipped with coal pulverizers, particularly once-through sliding pressure boilers.
In the past, it was common for electrical utilities to use oil and natural gas-fired peaking boilers to meet rapid changes in load demands. However, in light of our dwindling resources of oil and natural gas, electric utilities are now turning to coal-fired boilers to meet both their base load and peak demand requirements. Consequently, coal-fired boilers are more frequently being required to operate in what is termed a cycling mode.
In a typical cycling mode, the coal-fired boiler will operate weekdays at full load during the day and at minimum load at night. On the weekends, the unit would be removed from service. Accordingly, it is desirable that coal-fired power plants be brought from minimum load, typically about 20 percent of peak load, to peak load as quick as possible in order to meet the sudden rise in load demand which typically occurs as businesses, industries, and homes start their day. Additionally, it is desirous that the coal-fired boilers be able to quickly reduce load in the evening.
One of the major factors limiting the ability of coal-fired boilers to respond quickly to changes in load demand lies in the inability to get coal through the pulverizers into the boiler quick enough. In the operation of a typical coal-fired boiler, coal is fed to the pulverizer where it is finally ground and dried by hot air. The coal is then transported in an air stream through fuel pipes, often over 500 feet in length, through the burners of the furnace.
In order to increase or decrease the amount of coal fired in the furnace and thereby increase or decrease the load generating capacity of the furnace, the amount of coal being fed to the pulverizer must be, accordingly, increased or decreased. To change the amount of coal being fired to the pulverizer, it is common to change the speed of the feeder which feeds raw coal from the storage bin through the pulverizer. If the feeder speed is increased, the amount of raw coal fed to the pulverizer also increases. Conversely, if the feeder speed is decreased, the amount of raw coal fed to the pulverizer decreases.
However, the immediate increase or decrease in the supply of raw coal to the pulverizer does not result in an immediate change in the output of pulverized coal from the pulverizer to the burners. Rather there is a significant delay which is a major factor limiting the ability of a coal-fired boiler to respond to rapid changes in load demand. The reason for the delay is the coal storage capacity of the pulverizer which results from the residence time that the coal spends in the pulverizer in the grinding process. A change in the supply of raw coal to the pulverizer will result in an immediate change in the storage of coal within the pulverizer with a gradual delayed change in the rate of supply of coal to the furnace. The rate of supply of coal to the furnace will gradually change until a new equilibrium point is reached where the rate of coal supplied to the furnace is equal to the rate of input of raw coal to the pulverizer. Similarly, the reverse is true for a decrease of supply of raw coal to the pulverizer.
Because of this phenomenon, an operator is unable to rapidly change the rate of supply of coal to the furnace in order to generate the necessary heat to meet a rapid change in load demand. Rather, he must change the supply of raw coal to the pulverizer in a series of step changes with a pause between each step change to allow the rate of supply of pulverized coal to the furnace to match the rate of supply of raw coal to the pulverizer during that step. That is, he must delay his response to the rapid load change long enough to allow the pulverizer storage capacity to reach a new equilibrium point.